Wireless communications arc becoming more popular as technology develops. Many people are now communicating through mobile telephones, pagers, radio frequency (“RF”) devices and others. As this trend toward heavier usage of the frequency spectrum continues, there is an ongoing need to optimize the communication channels between the communication units in wireless telecommunications systems.
One optimization issue that arises with wireless communications is the need to maximize the throughput or utilization of a communication channel. For example, in the United States, the Federal Communications Commission (“FCC”) has allocated only a limited portion of the frequency spectrum for trunked private two-way RF communication. The reserved portion of the spectrum is divided into predefined frequency channels. Each communication channel generally requires a certain amount of available bandwidth to transmit substantive information (e.g., data, audio, video, multimedia, or some combination). Other factors being equal, the more of the transmitted signal that is used for the transmission of substantive information, the more efficient the utilization of the allocated bandwidth. In addition to the substantive information, however, there is also a certain amount of overhead information that typically must be transmitted in the communication channel.
One type of overhead information is control information that is exchanged between the communication units. For example, in a mobile telephone communication system, a base unit and terminal unit may exchange control information such as power control instructions, packet length arbitration, system ID information, service option, frequency, channel, gain, error, checking, and the like.
Another type of overhead information is synchronization information. Synchronization is generally utilized in coherent communications systems, wherein a unit that is initially operating asynchronously with respect to another unit is required to synchronize with the signals transmitted by the other unit. To synchronize, a receiving unit generally determines the timing of the information in a signal transmitted by a transmitting unit, and synchronizes its processing with the timing of the transmitted information. For example, in a mobile telephone communication system, a base station (or central cell or master or repeater) transmits a communication signal. Generally, a terminal unit (or roaming unit or subscriber unit) within range of the base station must acquire the transmitted signal before information can be exchanged. The terminal unit is initially operating asynchronously, and is not synchronized with the transmitted signal. As part of the signal acquisition process, the terminal unit generally has to align its frequency and timing with the transmitted signal.
In the prior art, specific synchronization information is generally inserted into the transmitted signal by the transmitting unit. The receiving unit uses this known synchronization information to determine the best timing at which to sample the incoming signal. Synchronizing with the incoming signal generally results in the receiving unit sampling at the timing interval at which there is the least interference from neighboring signals, thus establishing a reliable communication channel.
A prior art receiving unit typically uses a feedback loop to vary the frequency of its crystal oscillator to change the sample time until a signal lock is raised. The specific synchronization information of the prior art generally consists of a fixed symbol pattern in the transmitted signal. The receiver searches for these synchronization symbols by decoding the sampled potential symbol points into bits, and performing correlation over a large portion of the sampled signal until a symbol pattern in the sampled signal matches the fixed symbol pattern.
A problem with prior art systems is that the synchronization symbols are overhead information, and utilize part of the available bandwidth in the communication channel. This reduces the amount of bandwidth available for the transmission of substantive information, and thus reduces the maximum throughput of substantive information in the communication channel.
Another problem in the prior art is that the synchronization symbol values and patterns generally must be predetermined and programmed into both the transmitting unit and the receiving unit to enable the receiving unit to search for the same synchronization symbols that the transmitting unit is actually sending.
Because the synchronization symbols are inserted only periodically into the transmitted signal, another problem in the prior art is that the receiving unit generally only uses a small portion of the incoming signal for calculating the proper synchronization time, while the rest of the transmitted signal does not provide usable synchronization information. Thus the receiving unit may have to monitor the incoming signal over a significant period of time in order to receive sufficient information for synchronization.